Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B25/00—Phosphorus; Compounds thereof
  • C01B25/16—Oxyacids of phosphorus; Salts thereof
  • C01B25/26—Phosphates
  • C01B25/38—Condensed phosphates
  • C01B25/44—Metaphosphates
  • C01B25/445—Metaphosphates of alkali metals

Definitions

• step (b) Rapidly increasing the temperature of the resultant reaction mass from step (a) to about 560-850 C. to obtain additional potassium metaphosphate, said rapid temperature increase being at a sufficiently high rate to substantially obviate any formation of a pasty, sticky phase.
• This invention relates to an improved process for the preparation of potassium metaphosphate from phosphoric acid and potassium chloride.
• this reaction does not yield a monomeric metaphosphate KPO but a polyphosphate (KPO in which the degree of polymerization n varies according to operating conditions, values of n being reported as low as 2 and as high as 20,000.
• KPO polyphosphate
• metaphosphate is used for purposes of simplicity, it being understood that this salt is always obtained in a more or less polymerized form.
• Potassium metaphosphate is a binary fertilizer of special value owning to its very high content of nutritive elements available for plants: 60.1% P 0 and 39.9% K 0 for the pure product. Moreover, because of its low hygroscopicity, potassium metaphosphate has the outstanding advantage of not caking and of remaining free-flowing even under severe climatic conditions; thus, it can be readily handled and stored.
• potassium metaphosphate can be prepared from phosphoric acid and potassium chloride by a reaction at elevated temperatures, whereby a fluid molten product is obtained.
• a temperature higher than 900 C. with concentrated phosphoric acid but the severe corrosion attack caused by reaction products is such a serious drawback that it has heretofore not been possible to operate this process on an industrial scale.
• phosphoric acid and potassium chloride are reacted at the surface of a fused bath consisting essentially of potassium metaphosphate and contained in a tank furnace.
• the bath is maintained in the range of its melting point (between 560 and 850 C., preferably between 650 and 800 C.) by means of heat supplied partly externally to the liquid bath and partly to the interior of the bath by the Joule effect.
• the heating values of the gases leaving the tank furnace are recovered by sending these gases through heat exchangers before sending them to the HCl absorption plant.
• the potassium metaphosphate obtained by this above-mentioned process is in the molten form and its water solubility can be varied. It is well known, for example, that molten metaphosphate solidifies in a water-insoluble form when cooled down slowly, whereas it is partially or completely watersoluble when obtained by a more rapid cooling. In this way, various types of fertilizers can be prepared.
• the principal object of this invention is to provide an improved process for the production'of potassium metaphosphate.
• the process of the present invention comprises partly reacting phosphoric acid and potassium chloride in a first step by heating the reaction mixture at a temperature ranging from about 300 C., then completing the reaction in a second step by rapidly increasing the temperature of the partly reacted mixture to about 560-850 C., the potassium metaphosphate obtained in the molten form being then cooled until it becomes solid.
• the calories contained in the hot gases coming from the second step are preferably used to supply the total quantity or at least the major part of the heat required for reaction in the first step.
• the gases discharged from the second step are cooled, and their temperature being fairly low, they can be sent directly to the hydrogen chloride absorption unit without any further cooling.
• the phosphoric acid and potassium chloride are introduced together or separately, preferably in the form of a chloride suspension in the acid, into a first enclosure heated at about 120-300 C.
• this enclosure may be a rotary kiln heated by the gases coming from the second reaction step, and optionally with make-up heat supplied by a burner which allows an easier regulation of the temperature and thereby abetter control of the reaction.
• This first step of the reaction may also be performed in a tank furnace.
• the reaction yield obtained in the first step is advantageously comprised between about and 75%, preferably about 50 to 60%, the reaction being all the more rapid and complete when the temperature is higher within these limits.
• the residence time for the reactants ranges between about 15 minutes and 2 hours, preferably between about minutes and one hour.
• the partly reacted fluid mixture obtained in the first enclosure flows into a second furnace and the gases containing hydrogen chloride evolved during the reaction are sent directly to a usual absorption device for the recovery of an aqueous hydrogen chloride solution which is a valuable by-product.
• the fluid mass leaving the first furnace is introduced into a second furnace containing a liquid bath of metaphosphate maintained liquid by heating at a temperature comprised between about 560-850 C.
• the feeding is carried out in such manner that the temperature of the partly reacted mixture is sharply increased so as to avoid any significant local drop of temperature of the fused bath. This is accomplished by any conventional technique, for example, by introducing directly a relatively small quantity of partly reacted mixture into the bath of liquid metaphosphate contained in the second furnace.
• reaction mass from the first step is increased in temperature rapidly, and that a sufliciently high rate is employed to substantially obviate any formation of a pasty, sticky phase.
• a rate of temperature increase of about 0.5 to about 2 C./second is employed.
• the second furnace is heated by any suitable means, e.g., either directly by a burner placed between the bath surface and the furnace roof, or by Joule etfect using the fused salt as electrical resistance, an electrical voltage being applied between electrodes suitably spaced from one another and immersed in the bath, or by a radiant roof, or by any of these usual means combined.
• the second furnace may be a tank furnace or a rotary furnace. In this furnace the reaction is completed, the average residence time being at least about 20 minutes, preferably about to 80 minutes, longer times being permissible but unnecessary.
• the over-all yields are in the range of about 92 to about 98%.
• the molten potassium metaphosphate leaving the furnace at high temperature solidifies by cooling according to any suitable method. If the cooling is slow, the final product is water-insoluble. If the cooling is rapid, for example by tapping the molten potassium phosphate on a metal conveyor belt or on a rotary drum, one or the other being water-cooled, a totally Water-soluble product may be prepared. The solid metaphosphate is then ground and screened to suitable size for its final uses.
• the molten product may also be treated, according to the known method, by spraying it into air so that both cooling and granulating operations are effected simultaneously.
• any grade of KCl such as a commercial grade potassium chloride containing 58-60% K 0, and as the other reactant any phosphoric acid, e.-g., orthophosphoric acid prepared by electrothermal or by wet processes, metaphosphoric acid, superphosphoric acid, etc.
• any phosphoric acid e.-g., orthophosphoric acid prepared by electrothermal or by wet processes, metaphosphoric acid, superphosphoric acid, etc.
• substantially pure potassium metaphosphate melts at about 800 C. It is also known that the presence of certain compounds, such as potassium chloride, potassium sulfate, metal oxides (CaO, MgO, Fe O A1 0 SiO and metal salts (CaCl MgCl etc.) reduces this melting point significantly. For example, a mixture of metaphosphate-chloride containing 27% KCl melts at 610 C., and a mixture of metaphosphate-sulfate containing 13% K melts at 660 C.; and by adding topotassium metaphosphate both potassium chloride and sulfate in proportions that the mixture contains 32% KCl and, 6% K 80 the melting point decreases down to 560 C.
• certain compounds such as potassium chloride, potassium sulfate, metal oxides (CaO, MgO, Fe O A1 0 SiO and metal salts (CaCl MgCl etc.) reduces this melting point significantly.
• the jmetal oxides which are present as impurities in wet process phosphoric acid are capable of lowering the meltin g temperature of potassium metaphosphate down to about 700 C. Without further addition of adjuvants.
• the total and relative concentration of nutritive elemerits in the desired fertilizer determines the composition of the reaction mixture, and as explained above, the operating temperature of the second furnace depends on this composition.
• a fertilizer having the highest possible concentration in nutritive elements When a fertilizer having the highest possible concentration in nutritive elements is desired, equimolar proportions of acid and chloride are used. In contrast, to prepare a fertilizer having a lower total concentration but a higher K O/P O ratio (for example, about 1), there are several ways of proceeding. By using a mixture of acid-chloride containing a chloride excess, depending on the desired K O/P O ratio, a fertilizer consisting essentially of a mixture of potassium metaphosphate and chloride is obtained. Sulfuric acid may also be added to the raw materials and then a fertilizer containing potassium metaphosphate, potassium sulfate, and optionally some chloride is prepared.
• Example 1 A mixture of phosphoric acid containing 47.5% P 0.15% S0 0.12% F, and 2.6% Na O, and potassium chloride containing 61.3% K 0 and 1.6% Na O is prepared in a separate vessel, the proportions of acid and chloride being such that the mixture contains 1 mol of P205 per mol Of (K20+Nfl20).
• the slurry is introduced into a rotary kiln heated chiefly by convection-radiation by means of hot gases coming from a high-temperature tank furnace, Any additionally required heat is supplied by a burner, the rate of heat therefrom being easy to control by regulating the fuel feed; thus, the temperature in the kiln can be maintained at about 250 C.
• the average residence time of the reactants in the kiln is about 1 hour.
• the fluid mass When normal operating conditions are reached, the fluid mass is discharged from the rotary kiln to a tank furnace having a rectangular hearth, walls made of silicaalumina bricks, and electro-melted refractory material at the points more susceptible to corrosion.
• the introduction of the fluid mass is effected in the vicinity of one end .of the tank furnace.
• the latter is externally thermally insulated only on those parts which do not come in contact with the fused bath.
• a tap-hole is provided to discharge the molten product which overflows from the fused bath, the overflow level determining the depth of the bath.
• the major part of the heat is supplied to the tank furnace by a burner placed horizontally between the roof and the surface of the liquid bath at the end Where the partly reacted fluid mass is fed. Additional heat is supplied to the bath by applying an electrical voltage between four graphite bars places two by two at each end of the bath and totally immersed therein to prevent them from being too quickly deteriorated by the furnace atmosphere. It is in this tank furnace that the reaction is completed.
• a fused bath is prepared from previously manufactured potassium metaphosphate, the slurry of chloride and acid being then fed to the fused bath, When normal operating conditions are reached, the bath temperature is maintained at about 750 C. and the hot gases leaving the furnace are sent to the rotary kiln.
• An average sample of the product obtained contained: 59.95% P 0 and 32.60% K 0.
• the product cooled down slowly is water-insoluble.
• the quenched product is totally water-soluble.
• Example 2 Using phosphoric acid containing 47.6% P 0 2.7% Na O, 0.14% F, and 0.17% S0 and potassium chloride containing 60.85% K 0 and 1.85% Na O, a substantially equimolar slurry is prepared.
• the suspension is fed continuously to a rotary kiln heated chiefly by gases having an average temperature of about 900 C. coming from the second operating step, and by means of an auxiliary burner which make it possible to regulate the temperature so that it is maintained at 240-245 C. during the entire operation, the temperature being measured in the vicinity of the outlet of the kiln.
• the samples taken at regular intervals from the fluid mass leaving the kiln show an average yield of about 6 55-56% of initial KC] introduced in this first step.
• the residence time of the reactants in the kiln is about 45 minutes.
• the fluid mass leaving the rotary kiln is fed continuously onto a fused bath of potassium metaphosphate contained in a tank furnace having a rectangular hearth, heated with' a gas burner, and walls made of silica-alumina bricks with: electro-melted refractory bricks only where the surfaceof liquid bath comes in touch with the walls. Externally, ;the furnace is completely heat insulated.
• the bath temperature is maintained at about 705 C., and the hot gases, leaving the furnace at an average temperature of 900 C. are sent to the rotary kiln.
• the average production obtained was about 42 kg. per hour. .
• the melted potassium metaphosphate was cooled down moderately and an average sample thereof contained 59.15% P 0 (of which 23.75% were watersoluble), 31.45% K 0, 4% Na O, and 1.8% chlorine.
• the gases with an average temperature of 280 C. were sent to a hydrogen chloride absorption device.
• a process for the production of potassium metaphosphate which process comprises the successive steps of:
• step (b) introducing the resultant mass from step (a) into a molten bath of metaphosphate, said bath being maintained at a temperature of about 560-850 C. to rapidly increase the temperature of the resultant reaction mass from step (a) to about 560-850 C. to obtain additional potassium metaphosphate, said rapid temperature increase being at a sufliciently high rate to substantially obviate any formation of a pasty, sticky phase.
• step (a) is conducted at about 200-250" C.
• step (b) is about 650-800" C.
• step (a) is about ZOO-250 C.
• step (b) is about 650-800 C.
• step (b) The process of claim 1 wherein the phosphoric acid contains at least 25% of P 0 11.
• step (b) A process as defined by claim 1 wherein the rate of temperature increase in step (b) is about 0.5 to about 2 Centigrade degrees per second.
• step (b) A process as defined by claim 2 wherein the rate of temperature increase in step (b) is about 0.5 to about 2 Centigrade degrees per second.
• step (b) A process as defined by claim 3 wherein the rate of temperature increase in step (b) is about 0.5 to about 2 centigrade degrees per second.
• step (b) A process as defined by claim 4 wherein the rate of temperature increase in step (b) is about 0.5 to about 2 centigrade degrees per second.
• step (a) A process as defined by claim 1 wherein said potassium chloride in step (a) is reacted to the extend of about 20-75%, and that the reaction time of step (a) is about between minutes and 2 hours.
• step (a) is reacted to the extent of about -75%, and that the reaction time of step (a) is about between 15 minutes and 2 hours.
• step (a) is reacted to the extent of about 2075%, and that the reaction time of step (a) is about between 15 minutes and 2 hours.
• step (a) is reacted to the extent of about 5060%, and that the reaction time of step (a) is about between minutes and 1 hour.
• step (a) is reacted to the extent of about -60%, and that the reaction time of step (a) is about between 30 minutes and 1 hour.
• a process for the production of potassium metaphosphate which process comprises the successive steps of:
• step (b) introducing the resultant reaction mass from step (a) into a molten bath of metaphosphate, said bath being maintained at a temperature of about 560- 850 C. to rapidly increase the temperature of the resultant reaction mass from step (a) to about 560- 850 C. to obtain additional potassium metaphosphate, said rapid temperature increase being at a sufiiciently high rate to substantially obviate any formation of a pasty, sticky phase; and
• step (c) passing hot HCl-containing by-product gas from step (b) in heat exchange with the reactants in step (a).

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Glass Compositions (AREA)
• Fertilizers (AREA)

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Cited By (3)

Citations (2)

• 1963
  • 1963-08-07 FR FR944030A patent/FR1377348A/fr not_active Expired
• 1964
  • 1964-06-24 DE DEP34577A patent/DE1227875B/de active Pending
  • 1964-06-29 SE SE7929/64A patent/SE302117B/xx unknown
  • 1964-06-30 NL NL6407414A patent/NL6407414A/xx unknown
  • 1964-07-07 AT AT582464A patent/AT246100B/de active
  • 1964-07-10 ES ES0301929A patent/ES301929A1/es not_active Expired
  • 1964-07-15 LU LU46536D patent/LU46536A1/xx unknown
  • 1964-07-20 US US383990A patent/US3414375A/en not_active Expired - Lifetime
  • 1964-07-22 DK DK363464AA patent/DK106183C/da active
  • 1964-07-30 FI FI1634/64A patent/FI43740B/fi active
  • 1964-07-30 OA OA50223A patent/OA00179A/fr unknown
  • 1964-08-06 GB GB31999/64A patent/GB1065778A/en not_active Expired
  • 1964-08-07 BE BE651569D patent/BE651569A/fr unknown

Patent Citations (2)

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